In recent years, price competition of optical disc recording/reproducing apparatuses has heated up, and the demand for cost reduction of electronic parts such as LSI have been extremely increased. Effective means to achieve such cost reduction include an increase in integration density by process miniaturization and a reduction in space by reducing analog circuits.
Generally, a guide groove for guiding a beam spot of a light beam during data recording is formed on a recordable optical disc, and data are recorded along this guide groove. The guide groove is wobbled with a specific frequency to generate a reference clock for controlling the number of rotations of the disc. There is a land/groove recording system DVD-RAM in which data is recorded in each of a land and a groove, as one of large-capacity recording media. A land track and a groove track are alternately replaced with each other to form a single track from the inner circumference of the disc toward the outer circumference thereof. Further, the land track and the groove track are composed of sectors that are divided at a predetermined length, and a header region formed of pits, which is called CAPA (Complementary Allocated Pit Address), is located at the beginning of each sector. This pit sequence is located at an intermediate position between the land track and the groove track as shown in FIG. 11(a). The header region comprises a VFO (Variable Frequency Oscillator) and sector addresses as shown in FIG. 12. VFO1 in Header1 and Header3 is recorded with a single frequency, and it is used for pulling in a PLL (Phase Locked Loop). The disc is divided into several zones in its radial direction, and the number of sectors per track in each zone is constant. The number of sectors per track is increased from the zone at the inner circumference toward the zone at the outer circumference.
Data recording and reproduction should be performed after the number of rotations of the disc is controlled to the number of rotations suited to each zone. Accordingly, the linear velocity in each zone is approximately constant. A region other than the header region is a recording region in which data can be rewritten.
Detection of a track deviation amount for tracking control will be described. Although this track deviation amount is obtained from the reflected light from the disc, generally tracking servo is performed using a push-pull tracking error signal (hereinafter “tracking error” is referred to as “TE”) which occupies a relative positional error between the spot of the light beam and the center of the track. The TE signal includes not only a residual position error but also a lens shift disturbance corresponding to a relative positional deviation between the optical pickup body and the objective lens, a difference between the inner and outer circumferences due to a radial tilt, an error due to learning accuracy, and the like. As a means for correcting the lens shift disturbance, there is a method of generating an off-track error signal indicating a relative positional error between the beam spot of the light beam and the CAPA to enhance the accuracy of tracking servo. For example, a deviation between the beam spot and the track can detected by detecting a difference between the VFO1 signal amplitudes of the header 1 and the header 3 in the total reflection light amount signal (refer to Patent Document 1). FIG. 13 shows an example of a configuration of a conventional optical disc recording/reproducing apparatus.
The conventional optical disc recording/reproducing apparatus shown in FIG. 13 comprises an optical pickup 101, an adder 104, a variable amplifier 105, an offset voltage adjustment circuit 106, a switch 107, a HPF 108, a full-wave rectifying circuit 109, a LPF 110, a gate generation circuit 111, sample/hold (S/H) circuits 112 and 113, and a difference signal circuit 114. The optical pickup 101 comprises a light-receiving element 102 which is divided into four areas along the track tangential direction and the disc radial direction, and converts the received light amounts detected by the respective areas into current values, and four I/V conversion amplifiers 103 which convert the current values supplied from the respective areas of the light-receiving element 102 into voltage values.
A description will be given of the operation of the conventional optical disc recording/reproducing apparatus configured as described above.
The TE signals outputted from the pickup 101 are added by the adder 104, and the resultant amplitude of the header region is adjusted by the variable amplifier 105. At this time, the signal of the amplitude-adjusted header region is adjusted by the offset voltage adjustment circuit 106 so as to fall within the dynamic range.
On the other hand, a gate signal for extracting a reproduction signal in the header region is generated by the gate generation circuit 111, and the reproduction signal in the header region is extracted from the output signal of the variable amplifier 105 by the switch 107. Then, the extracted signal in the header region is AC-centered by the HPF 108, and the AC-centered reproduction signal in the header region is full-wave-rectified by the full-wave rectifying circuit 109. The full-wave-rectified reproduction signal in the header region is integrated by the LPF 110 to be input to the S/H circuits 112 and 113.
In the header-region signal that is integrated by the LPF 110, the amplitude data in the Header1 region is sampled and held by the S/H circuit 112, while the amplitude data in the Header3 region is sampled and held by the S/H circuit 113.
A difference signal between the Header1 amplitude data and the Header3 amplitude data which are sampled and held by the S/H circuits 112 and 113, respectively, is obtained by the difference signal circuit 114, and this difference signal is outputted as an off-track error signal. The off-track error signal can be used for tracking servo control including lens shift disturbance.
In this way, the off-track error signal for detecting a deviation between the beam spot and the track is extracted.    Patent Document 1: Japanese Published Patent Application No. 2001-6190